6 - Overall ATP yield from Glucose under Aerobic Conditions

What is the overall ATP yield from one molecule of glucose under aerobic conditions?

The actual ATP yield is approximately 30–32 ATP per glucose molecule.

• Glycolysis: 2 ATP (net) + 2 NADH

• Pyruvate → Acetyl-CoA: 2 NADH

• TCA Cycle: 2 ATP (GTP) + 6 NADH + 2 FADH₂

• Final yield depends on the NADH shuttle used to bring cytoplasmic NADH into mitochondria.

Why can’t NADH produced in the cytoplasm (during glycolysis) directly enter the mitochondrion?

The inner mitochondrial membrane is impermeable to NADH, so shuttle systems transfer reducing equivalents into the matrix.

What are the two major shuttles for transferring cytoplasmic NADH into mitochondria?

• Malate-Aspartate Shuttle

• Glycerol 3-Phosphate Shuttle

How does the Malate-Aspartate Shuttle work?

• Transfers electrons from NADH to malate, which crosses the inner membrane.

• In the matrix, malate is reoxidized to oxaloacetate, reducing NAD⁺ to NADH.

• Yields full 2.5 ATP per NADH (same as mitochondrial NADH).

• Used in heart, liver, and kidney.

How does the Glycerol 3-Phosphate Shuttle work?

• Transfers electrons from NADH to dihydroxyacetone phosphate (DHAP) to form glycerol 3-phosphate, which donates electrons to FAD in the inner membrane.

• Electrons enter ETC at Complex II (like FADH₂).

• Yields only 1.5 ATP per NADH.

• Used in skeletal muscle and brain.

Why do the two shuttles result in different ATP yields per cytoplasmic NADH?

• Malate-Aspartate Shuttle results in NADH in the matrix → full ETC energy output.

• Glycerol 3-Phosphate Shuttle delivers electrons to FAD, bypassing Complex I → lower proton pumping → less ATP.

What if Complex III is inhibited (e.g., by antimycin A)? (4)

• Electron flow is blocked between CoQ and cytochrome c.

• Complexes upstream (I and II) remain reduced; Complex IV stays oxidized.

• Proton pumping halts beyond Complex III → ATP synthesis declines or stops.

• Oxygen consumption drops significantly.

What if oxygen (O₂) is not available? (4)

• Complex IV cannot transfer electrons to O₂.

• All ETC components become fully reduced, blocking electron flow.

• No proton gradient → ATP synthase ceases function.

• Cells must rely on anaerobic glycolysis for ATP.

What if the inner mitochondrial membrane becomes permeable to protons (e.g., via uncouplers)? (4)

• Proton gradient dissipates.

• Electron transport continues (O₂ is consumed).

• ATP synthesis drops because proton-motive force is gone.

• Energy is lost as heat (used in brown fat thermogenesis).

What if Complex I is inhibited (e.g., by rotenone)? (3)

• NADH cannot donate electrons.

• ETC still functions using FADH₂ via Complex II.

• Overall ATP yield drops due to loss of NADH input.

What if ATP synthase (Complex V) is blocked (e.g., by oligomycin)? (3)

• Protons accumulate in the intermembrane space.

• Proton gradient becomes too steep → ETC slows/stalls.

• No ATP produced → cell energy crisis unless glycolysis compensates.

What if the adenine nucleotide translocase (ANT) is inhibited? (3)

• ATP cannot exit the matrix; ADP cannot enter.

• ATP builds up in mitochondria; ADP shortage stalls ATP synthesis.

• Proton gradient may remain intact, but no ATP exported to the cytosol.